In a transmission system that uses optical fibers it is possible to achieve a transmission capacity of 40 Gbit/s (gigabits per second) using a single wavelength. The transmission scheme (especially for modulation and demodulation) commonly used in this type of transmission system is the simplest scheme and is known as IM-DD (Intensity Modulation-Direct Detection). In this scheme, a transmitter performs on/off modulation of the optical intensity in accordance with digital 0 and 1 signals. A receiver converts the optical intensity into electrical amplitude signals by direct photoelectric conversion, and reproduces the signals by determining using a decision circuit whether a signal is 0 or 1 based on a predetermined threshold value. Here, the threshold value set in the decision circuit is normally fixed when the decision circuit is operating, and there are two methods for deciding this threshold value. One method involves optimization while the decision circuit is not connected to the optical fiber transmission path (so called back-to-back configuration), while the other method involves optimization while the decision circuit is connected to the optical fiber transmission path. In commercial systems and the like, the former method is employed because of its wide application range.
In the decision circuit of the receiver in an optical transmission system, wrong identifications of 0 and 1, namely, bit errors occur due to chromatic dispersion in the optical fiber and Amplified Spontaneous Emission (ASE) optical noise in an optical amplifier located not only on the transmission path, but also in transmitter or receiver. In high speed transmissions of 40 Gbit/s, these bit errors are a factor in limiting the transmission distance and, therefore, measures to counter them are necessary. These countermeasures can be roughly categorized into two types. The first type is forward error correction (FEC), and the second type is decision feedback equalization (DFE). Note that, in the most widely used error correction technology (ITU-T recommendation G.975), the threshold value of the decision circuit is fixed.
When performing error correction on the receiving side, firstly, predetermined calculation processing is performed on the transmission information on the transmitting side, and redundant bits obtained as a result thereof are attached and transmitted. Accordingly, there is normally an increase in the bit rate. On the receiving side, the predetermined calculation processing is performed on the received signals again, the result of this calculation processing is compared with the received redundant bits, and an error position is detected and a correction made by exclusive OR.
In contrast, the decision feedback equalizer method is proposed as a method for dealing with burst errors (F. Buchali et al., “Adaptive Decision Feedback Equalizer for 10 Gbit/s Dispersion Mitigation”, ECOC' 00, 5.2.5). In this method, there is one structure in which the bit error rate (BER) of decision reproduced signals or the like is measured, and the threshold value of the decision circuit is variably controlled such that this output bit error rate is at the minimum. There is also another structure in which the optimum decision circuit is selected from among a plurality of decision circuits in each of which a different threshold value is set.
However, the above described technologies have the following drawbacks.
FEC has the problem that the bit rate of the transmission signals is raised. For example, in super FEC, which is currently being examined in submarine transmission systems, 7% redundant bits and 12% redundant bits are connected in serial, so as to create a total of 22% redundant bits (O. A. Sab, “FEC Techniques in Submarine Transmission Systems”, OFC '01, TuF-1). However, in a high speed transmission such as 40 Gbit/s, the bit rate is approximately 49 Gbit/s, and the possibility arises that the speed margin of the electronic circuit will not be sufficient. Moreover, an interleaving circuit that mixes up the bit transmission sequence is used between the two encoding circuits, so that the circuitry size is increased. Furthermore, because some iteration decoding technology is used, the decoding delay cannot be ignored.
In this way, in FEC, there is a trade off relationship between the transmission signal bit rate and the coding gain (the forward error correction performance), and a way to obtain the maximum coding gain with the minimum rise in the bit rate is being investigated. In addition, FEC is effective against random errors, but is not particularly effective against burst errors. For example, in the gradual changes that occur in phasing phenomena such as polarization mode dispersion, it is supposed that the bits of the majority of the code words are erroneous, however, in this case, error correction performance is limited (M. Tomizawa et al., “FEC Performance in PMD-Limited High-Speed Optical Transmission Systems”, ECOC '00, 5.2.4).
In contrast, the drawback with decision feedback equalization (DFE) is that because the structure uses a feedback control circuit, there is a lengthy processing time and it is not possible to track rapid changes, e.g., within a bit-period. For example, in order to measure the bit error rate of a result output from a variable decision circuit (or selected output results from a plurality of decision circuits), and set (or select) a suitable threshold value, a control time corresponding to between several bits and several hundred bits is needed. Accordingly, DFE is effective against phasing, but is not effective against random errors, caused by noise or the like, in which there is no correlation between bit-errors.
Note that the above description is for an optical transmission system, however, the description also applies in the case of general wired or wireless electrical digital signal transmissions.